The present invention relates to a method for driving a piezoelectric ink jet head which selectively deposits ink droplets onto an image recording medium.
Of nonimpact printers, printers based on ink jet technology are the simplest in principle and most suitable for color printing.
Among them, it can be said that the so-called drop-on-demand type that ejects ink droplets only when forming dots is predominant. Many types of drop-on-demand piezoelectric ink jet heads that use piezoelectric elements have been disclosed, representative examples including the Kyser type disclosed, for example, in Japanese Examined Patent Publication No. 53-12138, the multi-layer piezoelectric actuator type disclosed, for example, in Japanese Examined Patent Publication No. 68427, and the shear mode type disclosed, for example, in Japanese Unexamined Patent Publication No. 63-252750. 
In such a piezoelectric ink jet head, a piezoelectric element deformable by the application of a pulse waveform is mounted at least on a portion of the wall surface of an ink chamber which communicates at one end with a nozzle and at the other end with an ink reservoir, and ink is ejected by deforming this piezoelectric element.
The piezoelectric ink jet head is usually driven in the following way. First, a pulse waveform is applied to the piezoelectric element to deform the portion of the wall surface of the ink chamber in such a manner as to cause the internal volume of the ink chamber to increase and thereby draw ink into the ink chamber. Next, by either removing the voltage from the piezoelectric element or applying a pulse waveform opposite in polarity to the first applied pulse waveform, the portion of the wall surface of the ink chamber is deformed in a direction opposite to the direction in which it was first deformed. This causes the internal volume of the ink chamber to decrease, and an ink droplet is ejected. This driving method is generally known as draw and eject.
However, the sudden drop in the internal volume of the ink chamber causes a sudden rise in the internal pressure of the ink chamber, and ink meniscus protrudes outwardly from the nozzle, forming an ink column, the ink column being caused to protrude further outward in the shape of an elongated rod. At this time, oscillations caused by the ejection of the ink droplet remains on the meniscus. On the other hand, the elongated ink droplet disintegrates into two independent droplets, i.e., a main droplet and a satellite droplet. The higher the ejection speed of the ink droplet, the shorter the time required to form the main droplet. This causes a displacement in time relative to the time that the ink droplet is cut off; the result of this is that the ink droplet is liable to become elongated. Accordingly, in the above driving method, if the ink ejection speed is increased, the ink droplet is liable to disintegrate. As a result, the ink droplet is deposited on the printing surface, not as a single dot but as a gourd-shaped dot or as two dots.
According to the ink jet head driving method of the present invention, only main droplets are ejected while the ejection of satellite droplets is prevented and a decrease in the ink ejection speed is avoided.
According to the ink jet head driving method of the present invention, in driving the ink jet head to eject an ink droplet, first the driving voltage is lowered from its initial voltage value, then the driving voltage is rapidly raised to cause the meniscus to protrude outward, and the driving is stopped immediately after a main droplet is formed. In this condition, the driving voltage is lowered, thereby causing the meniscus to retreat and thus suppress satellite ejection. Next, the driving voltage is raised to cause the meniscus to return to its initial position.
More specifically, the ink jet head driving method of the present invention ejects ink by deforming, using a piezoelectric actuator, at least a portion of a wall surface of an ink chamber which communicates at one end with a nozzle and at the other end with an ink reservoir.
According to the above driving method, in a first driving stage, a voltage applied to the piezoelectric actuator is lowered from an initial voltage value (VH), thereby driving the actuator to deform in a direction that increases the internal volume of the ink chamber and thus causing a meniscus in the nozzle to deflect toward the inside of the ink chamber.
Next, in a second driving stage, the piezoelectric actuator is driven to deform in a direction that reduces the internal volume of the ink chamber, thereby causing the meniscus in the nozzle to deflect toward the outside of the ink chamber and thus ejecting an ink droplet.
The second driving stage terminates when a main droplet is formed with the meniscus protruding outwardly from the ink chamber through the nozzle, wherein the driving voltage at that time is lower than the initial voltage value (VH).
Next, in a third driving stage, the piezoelectric actuator is driven to deform in the direction that increases the internal volume of the ink chamber, thereby causing the meniscus in the nozzle to deflect toward the inside of the ink chamber.
Further, in a fourth driving stage, the piezoelectric actuator is driven to deform in the direction that reduces the internal volume of the ink chamber, thereby causing the meniscus in the nozzle to deflect toward the outside of the ink chamber.
According to the present invention, it is possible to eliminate satellite droplets without decreasing the ejection speed of the main droplets. Since this serves to reduce the amount of ink deposited, high resolution printing can be achieved.
Furthermore, since the ejected ink droplet does not disintegrate, the printed ink droplet does not spread or flow if the spacing between the head and the recording medium becomes wider or if the feed speed of the head, etc. is increased. This increases the freedom in design in determining the spacing and the feed speed. Moreover, even when there is an appreciable step on the recording medium, since each ink droplet is printed as a single dot on the recording medium, no unevenness occurs in the printed result and a uniform print quality can be obtained.